Recently, demand on lithium secondary batteries with high energy density is increasing rapidly and researches are ongoing on their performance from various aspects. A lithium secondary battery basically consists of a positive electrode or cathode, a negative electrode or anode, and an electrolyte and a separator provided between the two electrodes. For the cathode and the anode, use is made of a slurry prepared by dispersing and mixing an active material, a conductive material, a binding agent and, optionally, a plasticizer in a dispersion medium, which is applied on a current collector such as metal foil, metal mesh, etc. As an active material applied to the cathode, lithium-transition metal oxide complexes such as a cobalt-based oxide complex (Li1-XCoO2), a nickel-based oxide complex (Li1-xNiO2), a manganese-based oxide complex (Li1-XMn2O4), etc. are widely used.
The lithium oxide complexes used as the positive electrode material of a lithium secondary battery are generally prepared by mixing a compound serving as the major component of the positive electrode material for a lithium secondary battery (e.g., carbonate or oxide of Co, Ni, Mn, etc.) with a lithium compound (e.g., lithium carbonate) and heat-treating the mixture.
For example, Japanese Patent Publication No. H01-294364 discloses a method for preparing a lithium oxide complex, comprising saturating an aqueous solution of chloride of Ni and Co with carbon dioxide gas and adding an aqueous solution of sodium bicarbonate to coprecipitate carbonate of Ni and Co, washing the resulting precipitate with water and drying at 140° C. in the presence of argon gas, and mixing the resultant with lithium carbonate and heating in the air.
And, Japanese Patent Publication No. H11-307094 discloses a method for preparing a lithium oxide complex, comprising adding an aqueous solution of sulfate of elemental components except for lithium and an aqueous solution of ammonium bicarbonate to which a small amount of ammonia slowly to a reaction tank, inducing substantially uniform crystal growth in the shape of concentric sphere while maintaining the pH of the mixture solution in neutral region, and mixing the resultant carbonate complex with lithium hydroxide and calcinating under oxygen gas atmosphere.
At present, the materials used for the electrodes of the lithium secondary battery are generally prepared by the solid-state method. However, since this method involves physical mixing and pulverization processes, repeated sintering and pulverization are required to ensure excellent electrochemical properties and superior crystallinity. Accordingly, a lot of time and cost are spent for the associated processes. Although time and cost can be saved by decreasing the sintering time, in this case, the electrochemical properties of the cathode active material are severely deteriorated and electrochemical performance of the cathode active material becomes poor.
Thus, the need on a cathode active material for a lithium secondary battery having superior electrochemical properties that can be prepared in short time at low cost, without requiring repeated sintering and pulverization processes, is increasing.